High-whiteness hydrophobic precipitated silica with ultralow moisture absorption

ABSTRACT

The invention relates to hydrophobicized precipitated silicas that possess an extremely high whiteness and extremely low moisture absorption. The hydrophobic precipitated silicas are prepared, inter alia, by coating with silicone oil and oxidative heat treatment.

CONTINUING APPLICATION DATA

This application is a Continuation of U.S. application Ser. No.10/211,314, filed on Aug. 5. 2002, now U.S. Pat. No. 7,022,375.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a hydrophobic precipitated silica featuringextremely low water absorption, a high level of whiteness, andproperties of low thickening in silicone rubber formulations with a highreinforcing action in the silicone vulcanizates.

2. Discussion of the Background

The treatment of finely divided solids, metal oxides, and silicates withorganosilicon compounds, such as organopolysiloxanes, is known, forexample, from DE 30 85 905. The heat treatment process this entails iscarried out under an inert atmosphere of nitrogen. Additionally,hydrophobic silicates are known, for example, from DE 24 35 860, DE 2242 728, and DE 25 13 608.

In these documents, hydrophilic silicates and precipitated silicas arehydrophobicized by reacting them with organosilicon compounds. Examplesof hydrophobicizers used include organohalosilanes andorganopolysiloxane compounds.

DE 26 28 975 and DE 27 29 244 describe preparing hydrophobic silicas byreacting a hydrophilic precipitated silica featuring low waterabsorbency with silicone oil or dimethyldichlorosilane, respectively. Inthe process according to DE 26 28 975, the reaction is carried out withthe hydrophobicizer (silicone oil) being added to the dry precipitatedsilica; in the process according to DE 27 29 244, the hydrophobicizer(dimethyldichlorosilane) is introduced directly into the precipitatedsilica suspension. In both cases, the hydrophobicizing step is followedby heat treatment at elevated temperatures, specifically between 200 and400° C.

A disadvantage of this process is that the precipitated silica thushydrophobicized becomes discolored at the required process temperatures.The discoloration of this silica is particularly inconvenient when it isadded to silicone formulations; that is, when these hydrophobicprecipitated silicas are added to silicone rubber formulations or todefoamer mixtures based on silicone oil.

As a measure of the discoloration it is possible to use the value knownas reflectance. In measuring the reflectance, the diffuse reflectionpower of a sample is investigated. The higher the diffuse reflectionpower of a sample, the higher its reflectance and thus the higher thewhiteness of the sample.

Precipitated silicas generally have a reflectance of not more than 97%.

Discoloration occurs in particular when the precipitated silicas arestrongly hydrophobicized; that is, have a high methanol wettability andhigh carbon loadings. It is precisely these properties, however, whichare in many cases (in silicone rubber formulations, for example)desired.

A further disadvantage of the known processes is that only a limitedamount of hydrophobicizer can be attached covalently to the silica.Particularly in silicone rubber formulations, however, high carbonloadings are desired, since they permit decisive improvements in therheological properties, such as the thickening, i.e., low yield pointand low viscosity, of the compounds.

As a measure of the thickening, it is possible to utilize the DBPnumber. The DBP number indicates the absorption capacity of a silica forDBP. The measurement technique shows the amount of dibutyl phthalate, ing, on a sample of 100 g, at which a massive increase in force in thecompounder is observed.

It is also not possible to achieve high carbon loadings by usingdiorganodichlorosilanes, or hydrolysis products ofdiorganodichlorosilanes, or with corresponding diorganopolysiloxanes inexcess to the silanol groups present, since the totality of theorganosilicon compounds is no longer attached covalently to the silica.In hydrophobicized silicas for fractions of hydrophobicizing agent thathas not been covalently attached, there is a risk that these moleculesmay have a marked mobility, which in many applications can be verydetrimental (e.g., in silicone rubber applications for medical purposesor for articles that are safe in food contact, such as pacifiers, etc.).

A further disadvantage of the prior art processes is that the relativelylow carbon contents of less than 3.1% lead to hydrophobic silicas whichhave a strong thickening action in silicone rubber formulations. DE 2628 975 lists data on the testing of hydrophobic precipitated silica insilicone rubber formulations, in which the hydrophobic precipitatedsilica is used in increasing weight fractions. From these it is clearthat, at a level of just 15% of hydrophobic silica in the rubber, theself-leveling properties of the silica disappear and that, at 20%,flowable compounds are no longer obtained. All tables clearly indicatethat all of the mechanical properties are improved as the filler contentgoes up. It would therefore be desirable to prepare silicone rubberformulations which include high fractions of hydrophobic silicas, forimproving the mechanical properties, but which at the same time arestill flowable.

SUMMARY OF THE INVENTION

The present invention provides a precipitated silica that has a highcovalently attached carbon loading, low water absorption, alow-thickening effect with good reinforcer properties in silicone rubberformulations, and a high level of whiteness. Ideally the presentinvention hydrophobicizes silicas in such a way that the reflectance ofthe original silica is retained.

It has surprisingly been found that a hydrophobic silica having therequired properties can be obtained by distributing a polysiloxane on ahydrophilic silica with subsequent conditioning and oxidative heattreatment.

The present invention provides hydrophobic precipitated silicascharacterized by the following properties:

carbon content >3.1%; methanol wettability  >60%; reflectance  >94%;BET/CTAB ratio >1 and <3; DBP absorption <230 g/100 g; BET surface area50–110 m²/g; CTAB surface area   >30 m²/g; water vapor absorption at 30°C. and 30% 1.1 ± 0.2%; and relative humidity water vapor absorption at30° C. and 70% 1.4 ± 0.3%. relative humidity

The ranges of preference specified may be adjusted independently of oneanother.

The hydrophobic silicas of the invention may additionally, eachindependently of one another, be characterized by the followingproperties:

Sears number <1.6; pH 5–9; water content <2%; conductivity <150 μS; andloss on ignition >3%.The conductivity may be below 100, 60, 30, or even 20 μS.

The present invention further provides a process for preparinghydrophobic precipitated silicas, comprising the following steps:

-   -   a) preparing a mixture of an organopolysiloxane derivative and a        precipitated silica;    -   b) conditioning the mixture at from 10 to 150° C. for a period        of from 0.5 to 72 h; and    -   c) conducting oxidative heat treatment at more than 300° C. with        an oxidizing gas.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred embodiments of the invention will be described, withreference to the following figures, wherein:

FIG. 1 shows a steep methanol wettability curve representative of thoseobtained using the hydrophobic silicas of the present invention; and

FIG. 2 shows the methanol wettability curve of a conventionalhydrophobic silica.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The hydrophobic precipitated silicas of the invention feature thefollowing properties:

-   an extremely high whiteness (reflectance above 94%);-   no discoloration in air even at temperatures above 300° C.;-   an extremely low moisture absorption, at the same level as pyrogenic    silicas;-   a highly homogeneous hydrophobicization, i.e., a steep methanol    wettability curve;-   high methanol wettability (>60%);-   high level of firmly attached carbon (>3.1%);-   low thickening action in silicone rubber;-   contains virtually no ionic impurities, i.e., low conductivity <150    μS; and-   contains no surfactants, emulsifiers or organic solvents which might    lead to discoloration at elevated temperatures.

The process for preparing the silica of the invention makes it possibleto obtain homogeneous distribution of the hydrophobicizer while avoidingthe use of solvents (except for water), emulsifiers, surfactants orother surface-active molecular moieties in the hydrophobicizer, so thatthe resulting silica combines low-thickening properties with absence ofdiscoloration.

The effective distribution of the hydrophobicizer and the high degree ofhydrophobicization of the precipitated silica of the invention result insilicone rubber formulations, for example, with very low thickening,unimpaired even by prolonged storage, in conjunction with goodmechanical and optical properties in the vulcanizates.

The silica of the invention is preferably prepared with a polysiloxane,so that after heat treatment its only organic radicals are methylgroups, this going hand in hand with a very high thermal load-bearingcapacity (>300° C. with ingress of air does not lead to discolorations).

The process of the invention makes it possible to effecthydrophobicization particularly of silicas with a low silanol groupdensity (the measure used for the amount of silanol groups is the Searsnumber, i.e., the alkali consumption of an acid-based titration;together with the BET surface area it is then possible to comparesilanol group densities relatively) and to achieve a high whiteness(>94% reflectance) in the resulting silica of the invention.

The water absorption of the hydrophobic precipitated silica of theinvention, which is very low for a precipitated silica, is made possibleby the selection of a hydrophilic precipitated silica with a very lowsilanol group density and through very homogeneous hydrophobicizationwith organosilicon compounds. The measure used for the silanol groupdensity is the Sears number in proportion to the BET surface area.

The reaction conditions employed in the process of the invention do notlead to carbonization and thus lead to virtually no discoloration. Forthis reason it is important that the hydrophilic base silica contains noorganic impurities, since otherwise the discoloration increases. Sincethe cleavage products which form as a result of the heat treatment, andwhich are gaseous under the process conditions, may bring about acertain degree of discoloration even in an oxidizing atmosphere, it isimportant to remove these cleavage products from the product by means ofa sufficient throughput of gas.

The use of liquid polysiloxane, preferably the polydimethylsiloxane ofpreferably 30–100 cSt, permits optimum distribution on the base silica.Under the oxidative reaction conditions, the polydimethylsiloxanes usedare able to undergo resinification. This has the great advantage thatthe hydrophobicizer can be distributed on the silica in liquid form andthen fixed. The amount of bound carbon can be greatly increased by theoxidative heat treatment.

Silicas of the invention exhibit steep methanol wettability curves. Forexample, FIG. 1 exhibits a steep wettability curve obtained using asilica of the present invention that shows that homogeneoushydrophobicization has taken place. In contrast, FIG. 2 shows themethanol wettability of conventional hydrophobic silicas heat-treatedwithout the conditioning of the present invention.

High carbon loadings and high methanol wettabilities bring aboutdecisive improvements in the properties of silicas in silicone rubberformulations. The further reduction in moisture absorbency permitsvulcanizations at atmospheric pressure and at temperatures above 100° C.in silicone rubber formulations, since no disruptive vapor bubblesappear in the vulcanizate. The high carbon content silicas of theinvention exhibit substantially improved Theological properties insilicone rubber formulations; that is, they have only a low thickeningaction, and exhibit low yield points. This low thickening action makesit possible to prepare flowable silicone rubber formulations that arefilled with well above 20% of hydrophobic precipitated silica and yetcan still be processed by injection molding. Additionally, the higherfilling level leads to markedly improved mechanical properties in thevulcanizates.

The hydrophobic precipitated silica of the invention may therefore beused in the following applications.

1. As a Filler in Silicone Rubber Formulations

The silica of the invention can be used without furtherhydrophobicization in all types of silicone rubber. The low waterabsorption suppresses evolution of vapor in high temperaturecrosslinking systems such as LSR and HTV silicone rubber, and allowspore-free vulcanizates. The low DBP number leads to low compoundviscosities, which can be used to advantage in LSR and RTV2 compounds.The compounds possess a high level of stability on storage: thephenomenon referred to as afterstiffening is suppressed. Because of thelow moisture absorption, the silica of the invention can also be used insystems which cure by atmospheric humidity such as RTV1 compounds. Thecompounds likewise have a high level of stability on storage. Since themoisture content of the silica of the invention is greatly reduced, theunwanted hardening during storage is suppressed.

Because of the high whiteness of the silica, attractive whitevulcanizates can be produced. The low water absorption of the silicaresults in low moisture absorption in the vulcanizates. This produceshigh electrical resistance and an increase in aging stability,especially at high temperatures. These properties are particularlyuseful in electrical insulators and in seals.

2. As a Defoamer Component

It is known, for example, from DE 28 29 906, U.S. Pat. No. 4,377,493, DE34 11 759, U.S. Pat. No. 4,344,858, and WO 95/05880 that hydrophobicizedsilicas can be used in defoamer formulations. Advantageous forhigh-level defoamer performance here are the high hydrophobicity and ahigh surface area, readily accessible even to relatively largemolecules, of the silica of the invention. The high hydrophobicity ofthe silica of the invention, furthermore, ensures high alkali resistanceand results in much higher service lives particularly in stronglyalkaline media.

The high reflectances of the silicas of the invention ensure appealingdefoamer formulations free from discoloration, particularly informulations based on mineral oil and silicone oil.

3. As a Free-Flow Agent

It is known (for example, from Degussa AG brochure series,Fällungskieselsauren und Silikate (Precipitated silicas and silicates),1984) that hydrophobicized silicas can be used as free-flow auxiliaries.Because of its low water absorption, the silica of the invention isparticularly suitable as a free-flow auxiliary for substances that aresensitive to hydrolysis. Here again, the high reflectances of thesilicas of the invention are an additional advantage.

The silica of the invention may also be used as a carrier substance,particularly for insecticides, as an insecticide per se, has anantiblocking auxiliary, or filler in silicone rubber mixtures which cureby atmospheric humidity.

The hydrophobic precipitated silicas of the invention are prepared inthree steps.

First of all, a liquid polysiloxane derivative is initially distributed,physically, on the silica surface. Where this initial distribution iscarried out in aqueous media, i.e., suspensions or silica with a watercontent of more than 70%, the silica is typically unstable. It musttherefore be isolated quickly by filtration following the initialdistribution, and/or subjected to accelerated drying (in a spin-flashdrier or nozzle tower drier, for example). This conserves thedistribution of the organopolysiloxane droplets on the silica andprevents separation into water, silicone oil, and silica.

Subsequently, in a controlled conditioning step—process step b)—thedistribution of the hydrophobicizer is improved further and engagementof the polysiloxane derivative with the silica surface is achieved. Thisstate of distribution is stable even in aqueous media. Following processstep b), there is no longer any separation between the polysiloxanederivative and the silica. At carbon contents ≧3.1, the conditionedsilicas can be adjusted steplessly to a methanol wettability of up to55%. The BET/CTAB ratio after this step is <1. The binding of thepolysiloxane to the silica is thought to be a result of the formation ofmultiple hydrogen bonds between the siloxane bridges of the polysiloxanemolecules and the silanol groups on the silica surface.

This is followed by a heat treatment in an oxidizing atmosphere, whichsuppresses discoloration phenomena, ensures covalent binding of thehydrophobicizing agent, and—probably as a result of the formation ofgaseous cleavage products—increases further the distribution of thehydrophobicizer. Heat treated silicas, with a lower carbon content thanthe corresponding conditioned silica, have a higher methanolwettability. Heat treatment in an oxidizing atmosphere assists theresinification of the polysiloxanes, so that much larger amounts ofhydrophobicizer can be anchored covalently on the silica. The BET/CTABratio has turned around and is now >1.

As the organopolysiloxane derivative it is possible to use allorganosilanes or organohalosilanes that are commonly used tohydrophobicize precipitated silicas.

Step a) of the process of the invention can be conducted with thefollowing variants.

A first variant of step a) is the addition of organopolysiloxanederivative to a precipitated silica having a water content of from 1.0to 80% by weight, preferably from 20 to 60% by weight;

A second variant of step a) is the addition of the organopolysiloxanederivative to a dispersion of the precipitated silica, i.e., followingprecipitation of silicate with an acid, for example, using a Rhein-Hüttemixer or Kotthof-Mischsirene or Ultra-Turrax. This necessitates rapidfiltration and/or accelerated drying (spin-flash drier, spray drier,nozzle tower) after the reaction.

A third variant of step a) is the addition of the organopolysiloxanederivative to a precipitated silica having a water content of from 70 to99% by weight, with subsequent isolation of the solid from the water.Isolation can be effected by filtration, nozzle tower, spin-flash, orother short term drying. The higher the water content, the more quicklyisolation should be carried out. Separation should be avoided.

A fourth variant of step a) is simultaneously supplying the precipitatedsilica or hydrous silica and the organopolysiloxane derivative to aspin-flash drier.

A fifth variant of step a) is mixing of dry precipitated silica withpolysiloxane, in a Gericke mixer, for example.

An alternative possibility is first to prepare a masterbatch, i.e., aconditioned precipitated silica, obtained according to process steps a)and b), and to mix it with a (hydrophilic), hydrous precipitated silica(e.g., filtercake, silica suspension or silica dispersion).

The water content of the hydrophilic precipitated silica may vary withinthe ranges mentioned above.

The base silica may be coated in a mass ratio, for example, of from 1:1to 1:3 with silicone oil, e.g, DOW CORNING (R) 200 FLUID 50 CS (50 mPasdimethylpolysiloxane terminated with trimethylsilyl groups, carboncontent of approximately 33%) (step a)). The resulting powder isconditioned at a temperature of more than 100° C. for half an hour, forexample. The conditioning (step b) here is carried out until theresulting material is wettable by water (methanol wettability <20;regarding the definition of methanol wettability see the measurementtechnique section) but which when introduced into water no longerexhibits any separation between silica and silicone oil (if step c)follows directly on from step b), a methanol wettability >20 ispreferred). Mixing of this masterbatch, (e.g., 50% by weight silica and50% silicone oil) with aqueous silica dispersions or silica suspensionsproduces stable mixtures in which the silicone oil no longer separatesfrom the silica. The total mixture typically contains one part by weightof silicone oil, about 4–8 parts by weight of silica, and 20–60 parts byweight of water. In order to prepare such a suspension, for example, themasterbatch (e.g., 50% silica and 50% silicone oil) can be mixedthoroughly with about 10–16 times the amount of filtercake (solidscontent approximately 20%) and about 10–20 times the amount ofadditional water. The advantage of this procedure is that thewater-wettable masterbatch (which contains up to 75% of hydrophobicorgano polysiloxane) can be dispersed directly in silica precipitationsuspensions or silica feeds, very finely and stably, without the need touse emulsifiers or surfactants. After such a mixture has been dried, orfiltered and then dried, the organopolysiloxane-containing silica thusobtained can be conditioned again (step b).

These steps can be carried out individually, where appropriate withgrinding beforehand. Milling should not, however, be carried out beforecoating a). It is also possible to carry out two or more of thesevariants—that is, identical or different variants—in succession.

The following embodiments of the process of the invention areconceivable.

In a first embodiment, one of steps a), b), and c) is performed a numberof times (from 2 to 5 times) in succession.

In a second embodiment, steps a) and b) are carried out a number oftimes (from 2 to 5 times) in succession.

In a third embodiment, all steps a), b), and c) are carried out a numberof times (from 2 to 5 times) in succession; in other words, the processis run through a number of times.

Process step b) is preferably carried out by heat treatment at 100–150°C. over the course of from 0.5 to 2 hours. After conditioning, thepartly hydrophobicized silica present may have a methanol wettability of20% or more. Fundamentally, a distinction may be made between wet anddry hydrophobicization.

Wet hydrophobicization means that the silicate starting materials areaqueous silica suspensions, silica feeds, or high water content silicafiltercakes, which are coated with the corresponding hydrophobicizers,as described, for example, in DE 27 29 244 for precipitation suspensionswith organohalosilanes.

Dry hydrophobicization means that the silicate starting materials aresilica powders with different moisture contents of from 1 to 75%, whichare coated with the corresponding hydrophobicizers. This process isdescribed, for example, in DE 26 28 975.

The silica of the invention is prepared using organopolysiloxanederivatives; it is, however, also possible to use other siliconecompounds which react to give organopolysiloxanes under the chosenreaction conditions (for example, dichlorodimethylsilane in an aqueousenvironment).

Hydrophobicizing reagents used comprise organopolysiloxane derivativesor their precursors, for example, those with the compositionR_(4-n)SiX_(n) (where n=1, 2, 3), (SiR_(x)X_(y)O)_(z) (where 0≦x≦2,0≦y≦2, 3≦z≦10, with x+y=2), (SiR_(x)X_(y)N)_(z) (where 0≦x≦2, 0≦y≦2,3≦z≦10 with x+y=2), SiR_(n)X_(m)OSiR_(o)X_(p) (where 0≦n≦3, 0≦m≦3,0≦o≦3, 0≦p≦3, where n+m=3, o+p=3), SiR_(n)X_(m)NSiR_(o)X_(p) (where0≦n≦3, 0≦o≦3, 0≦m≦3, 0≦p≦3, with n+m=3, o+p=3),SiR_(n)X_(m)(Si_(x)X_(y)O)_(z)SiR_(o)X_(p) (where 0≦n≦3, 0≦m≦3, 0≦x≦2,0≦y≦2, 0≦o≦3, 0≦p≦3, 1≦z≦10000 with n+m=3, x+y=2, o+p=3). Thesecompounds may be linear, cyclic, and branched silane, silazane andsiloxane compounds. R may comprise alkyl and/or aryl radicals, which maybe substituted by functional groups such as the hydroxyl group, theamino group, polyethers such as ethylene oxide and/or propylene oxide,and halide groups such as fluoride. R may also contain groups such ashydroxyl, amino, halide, alkoxy, alkenyl, alkynyl, and allyl groups, andgroups containing sulfur. X may comprise reactive groups such assilanol, amino, mercapto, halide, alkoxy, alkenyl, and hydride groups.

Preference is given to linear polysiloxanes having the compositionSiR_(n)X_(m)(SiR_(x)X_(y)O)_(z)SiR_(o)X_(p) (where 0≦n≦3, 0≦m≦3, 0≦x≦2,0≦y≦2, 0≦o≦3, 0≦p≦3, 1≦z≦10000 with n+m=3, x+y=2, o+p=3) in which R ispreferably represented by methyl.

Particular preference is given to polysiloxanes having the compositionSiR_(n)X_(m)(SiR_(x)X_(y)O)_(z)SiR_(o)X_(p) (where 0≦n≦3, 0≦m≦1, 0≦x≦2,0≦y≦0.2, 0≦o≦3, 0≦p≦3, 1≦z≦1000 with n+m=3, x+y=2, o+p=3) in which R ispreferably represented by methyl. Owing to the chosen process of theinvention, however, it is specifically also possible to usepolysiloxanes of low volatility which contain no functional groups.

Because of the presence of certain functional groups inorganopolysiloxane derivatives, salts or low molecular mass substancessuch as NH₃, amines, alcohols, etc. may be formed, which can lead todisruptive impurities. An important exception here is constituted bysilanol-functionalized polysiloxanes, since the only impurity formedhere is water, which is easy to remove under the chosen processconditions.

With preference, the hydrophobicizer may comprise a methyl-terminatedpolydimethylsiloxane, in particular one having a viscosity of 30–100mPas, preferably 40–60 mPas. An example of a suitable polysiloxane oilis DOW CORNING (R) 200 FLUID 50 CS.

Since the aforementioned hydrophobicizers are compounds of lowvolatility, an important part in the initial distribution of thehydrophobicizers on the silica surface is played by capillary forces anddiffusion events at the liquid/solid phase boundary.

Even if the hydrophobicizers used with preference exhibit a certainvolatility in the course of a thermal treatment, the liquid/soliddistribution is still important. For this reason, a distinction is madehere between physical, initial distribution, conditioning, and heattreatment.

The heat treatment, i.e., process step c), is conducted at at least 300°C., preferably above 350° C., with very particular preference above360–370° C., with an oxidizing gas. This gas can be air, Cl₂, NO_(x)(NO₂, N₂O₅, NO, N₂O), O₃, O₂, Br₂, F₂, or a mixture of these gases withfurther inert gases such as CO₂, N₂ or burner waste gases, in each casepreferably at not less than 1% by volume.

Additionally, the oxidizing gas may optionally contain up to 80%,preferably up to 50%, with particular preference 20–40%, by volume ofwater.

In every case, a good gas throughput must be ensured; as far aspossible, the gas must reach every silica particle. Apparatus suitablefor ensuring this includes, for example, metal lattice ovens, mufflefurnaces, or belt reactors.

In a muffle furnace, the bed height should be from 1 to 2 cm.

It is possible to combine two or more of these types of reactor. Theprocess may be operated batchwise or continuously.

By way of example, the text below gives further details of theconditions in a belt reactor and in a muffle furnace.

Heat Treatment of a Granule Bed (in a Belt Reactor)

Heat treatment takes place with relatively high process gas throughputs(>5 m³/(h·kg), energy input via the process gases) through a bed ofpolysiloxane-treated silica. For this purpose, the silica must begranulated beforehand. Since the energy is supplied by way of theprocess gases, it is relatively easy to avoid local overheating here.Although the high process gas volumes result in a low concentration ofcrack products in the process gas, this does not result in poorhydrophobicization via the gas phase, since the concentrations of crackproducts in the granular pieces are much higher than between thegranular pieces. Nevertheless, excess crack products can be expelledreadily from the granular pieces, so that here as well it is possible toobtain high-whiteness products with optimum hydrophobicization. Thisreactor type can be operated safely at oxygen concentrations of 4–21%.Spherical granules are preferably of the order of 2–20 mm in size, whilefor rodlet-shaped granules widths of 2–20 mm and lengths of 2–40 mm arepreferred. A bed of granules can be heat treated batchwise orcontinuously (in a belt reactor).

The preferred heat treatment times are 0.15–3.5 h at temperatures of320–400° C., for a duration of 0.15–4 h.

The reactor size may range from laboratory scale (100 g range/charge)via pilot plant size (100 kg range/charge) through production scale(>100 kg/charge).

When using this type of reactor, the heat treatment is typicallyfollowed by grinding of the granules.

Heat Treatment in a Muffle Furnace

The polysiloxane-coated silica is heat treated preferably in flat trayswith a bed height of 2 cm. Heat treatment is carried out in air. The bedheight selected firstly ensures a sufficiently high concentration ofcrack products in the interior of the bed (which permits effectivehydrophobicization by means of the gaseous cleavage products), butsecondly permits the unhindered escape of excess crack products.

Heat treatment in a muffle furnace may take place at 330–380° C. for aduration of at least 15 minutes but not more than 4 hours.

Following the conditioning step and/or heat treatment, thehydrophobicized silica is optionally ground. Milling before the coatingstep a), however, is not appropriate, and leads to low-grade productswith inhomogeneous hydrophobicization.

Optional milling gives a silica having a d_(4.3) of 8–25 μm, preferably8–15 μm.

The use of the precipitated silicas of the invention as a filler insilicone rubber formulations, in elastomer mixtures, polymers (e.g.,PVC, polyurethanes), tires or sealants, as defoamer auxiliaries or asfree-flow auxiliaries is likewise provided by this invention.

In order to develop fully their mechanical properties, silicone rubberformulations require active reinforcing fillers. It is common to usehighly dispersed silicas. Because of the ease of mechanical processingof LSR (liquid silicone rubber) formulations, especially in injectionmolding processes, HTV (high temperature vulcanizing) silicone rubberformulations are increasingly being replaced by LSR mixtures. Thereinforcing filler must bring about good mechanical properties in thevulcanizate without impairing the rheological properties of the siliconerubber formulations. After compounding, the silicone rubber formulationsmust be flowable and should not undergo afterstiffening even followingprolonged storage times.

HTV and LSR formulations are processed at temperatures well above 100°C. At such temperatures, hydrous fillers may lead to disruptiveformation of vapor bubbles in the silicone formulation. In the case ofsilicone rubber formulations which cure by atmospheric humidity, anexcessively high water content in the filler results in unwanted curingin the course of storage. Accordingly, the water absorptioncharacteristics, i.e., the amounts of water adsorbed at differentrelative atmospheric humidities, constitutes a measure of theprocessability of the filler.

The problem of vapor bubble formation occurs particularly with thehydrophilic precipitated silicas. Even hydrophobic precipitated silicasdo not, typically, exhibit the low water absorption characteristics ofthe pyrogenic silicas.

The hydrophobic precipitated silica of the invention, however, exhibitswater absorption characteristics comparable with those of pyrogenicsilicas, is unaffected by discoloration, and also has low-thickeningproperties in silicone rubber formulations.

These properties are derived from the nature of the base silica used andfrom the nature of the hydrophobicization. The base silica is preferablya precipitated silica which has a very low silanol group density (themeasure used for the silanol group density is the Sears number takentogether with the BET surface area). The low silanol group density ofthe base silica is also manifested in a low loss on ignition of 3.0±0.5at a BET surface area of about 160 m²/g.

For silicone rubber mixtures which are processed at temperatures ofalmost 200° C. with ingress of air, it is important that there are noorganic constituents on the silica which might undergo discolorationunder the influence of oxygen at these temperatures. Organosiliconcompounds containing exclusively methyl, phenyl, fluorocarbon orhydrofluorocarbons as organic radicals are extremely temperature-stableeven in the presence of atmospheric oxygen. In order, however, toachieve effective cleavage of the stable siloxane bridges of siloxanecompounds and to bond them covalently to the silica, temperatures above300° C. are required. At these high temperatures, siloxane compounds,especially in the case of precipitated silicas with a low silanol groupdensity, normally lead to discoloration phenomena on the silica. Theprocess of the invention makes it possible to suppress thisdiscoloration. These discoloration phenomena are measured by reflectancemeasurements with an optical measurement technique based on diffusereflection. Where the reflectances of silica are >94%, the silica-filledsilicone rubber compound appears pure white. Since the refractiveindices of silica and silicone rubber are close to one another, evenvery small impurities and discolorations in the silica filler becomeclearly visible in the silicone rubber. A reflectance of 93% alreadyleads to a marked discoloration in the silicone rubber, visible with thenaked eye, despite the fact that the silica powder before incorporationappears pure white to the viewer.

By mixing the silica of the invention with diorganopolysiloxanes and,where appropriate, further substances at room temperature or onlyslightly elevated temperature, it is possible to prepare compositionswhich can be cured to give elastomers, following the addition ofcrosslinking agents where appropriate. The silica of the invention has alow thickening effect, so that the compounds can be processed in LSRsystems on injection molding machines. Mixing can be carried outconventionally, in mechanical mixers, for example.

The fillers used in accordance with the invention are employedpreferably in amounts of from 1 to 50% by weight, more preferably from10 to 40% by weight, based on the overall weight of the compositionswhich can be cured to give elastomers. In the case of HTVorganopolysiloxane elastomers it is possible to use up to 50% by weight.

Besides diorganopolysiloxanes, the hydrophobicized precipitated silicaof the invention, crosslinking agents and crosslinking catalysts, thecompositions which can be cured to elastomers may of course whereappropriate include fillers which are conventionally, often or usually,used in compositions that can be cured to elastomers. Examples of suchsubstances are fillers having a surface area of less than 50 m²/g, suchas quartz flour, diatomaceous earth, and also zirconium silicate andcalcium carbonate, and also untreated pyrogenic silica, organic resins,such as polyvinyl chloride powders, organopolysiloxane resins, fibrousfillers, such as asbestos, glass fibers and organic pigments, solubledyes, fragrances, corrosion inhibitors, agents which stabilize thecompositions against the influence of water, such as acetic anhydride,agents which retard curing, such as benzotriazol, and plasticizers, andalso trimethylsiloxy-endblocked dimethylpolysiloxanes.

The cited combination of physicochemical characteristics of thehydrophobic precipitated silica of the invention results in anoutstanding reinforcing filler. The equilibrium moisture content, muchlower than that of the known precipitated silicas, brings advantages inprocessing, in the context, for example, of vulcanization at atmosphericpressure, which produces pore-free vulcanizates in comparison with theuse of the known, hydrated precipitated silicas. The optimized pH andthe low DBP number lead to perceptibly reduced roller-softening times.The low electrolyte content in combination with the low moisture contentleads ultimately to good electrical properties in the vulcanizates.

In silicone rubber sealants that cure by atmospheric humidity, the lowwater content of the hydrophobic precipitated silica of the inventiongives advantages for the storage properties of the uncured sealants.

The examples which follow are intended to illustrate the presentinvention, without restricting its scope.

EXAMPLES

As the silicate starting material, it is preferred to use precipitatedsilicas which possess a very low silanol group density, i.e., a lowalkali consumption/BET surface area ratio, a relatively high CTABsurface area for approximately the same BET surface area, and a highlevel of whiteness and purity.

Preparation of Base Silica

Aqueous sodium silicate solution (waterglass), and sulphuric acid isadded with stirring into a reaction vessel pre-charged with water.During the addition, an alkaline pH is maintained. The silicaprecipitates from the reaction mixture, which is then acidified to pH2–5 and filtered. The solid product is washed neutral with water anddried.

Any silica with the following properties is suitable as startingmaterial for the hydrophobic process according to the invention.

BET surface area (m²/g) 50–170 CTAB surface area (m²/g) 50–170 Loss onignition based on the substance dried ≦3.5% 2 h/105° C. (DIN 55921) (%)pH 5% (methanol/aqueous solution) (DIN 53200)  5–9 Conductivity (in 5%aqueous dispersion) (μS) <500 μS Tapped density (g/l) >200 g/l SearsNumber <13

The base silica and the polysiloxane are mixed until a defined carboncontent is obtained; in other words, the mixing ratio is a function ofthe arithmetic proportion for setting the required carbon content.

1. Measurement Techniques

1.1 Methanol Wettability

Silicas whose surfaces have been modified with nonhydrolyzable organicgroups are usually not wetted by water.

These hydrophobic silicas can, however, be wetted by a methanol/watermixture. The fraction of methanol in this mixture—expressed as apercentage by weight—is a measure of the hydrophobicity of modifiedsilica. The higher the methanol fraction, the better thehydrophobicization of the substance.

Procedure:

200 mg of each hydrophobic silica or silicate sample is weighed out into6 centrifuge tubes each with a capacity of 15 ml, and each of the tubesis filled with 8 ml of a methanol/water mixture of ascending methanolconcentration. The methanol concentration of the mixtures is guided bythe anticipated methanol wettability. The centrifuge tubes are tightlysealed and then shaken vigorously (10 up-and-down motions). To separatethe wetted silica/silicate fractions, the tubes are then centrifuged at2500 rpm for 5 minutes. The wetted fractions form a sediment whosevolume can be read off on the scale on the centrifuge tubes. On a graph,the sediment volumes are plotted against the methanol/water mixtureconcentration. These individual points produce a curve whose positionand steepness characterizes the degree of hydrophobicization of thesample under analysis.

Apparatus:

Precision balance

Centrifuge

Centrifuge tubes, graduated

Dispensettes

1.2 DBP Absorption

The DBP absorption (DBP number), which is a measure of the absorbency ofthe precipitated silica, is determined as follows:

The dibutyl phthalate number is determined using the Brabenderplastograph. The DBP number is a measure of the absorbency of apulverulent product for liquid. The absorbency is dependent on themoisture content, the particle size, and the amount of materialanalyzed.

Apparatus and Reagents

Brabender plastograph with plotter

Multi-Dosimat E 415 (50 l) from Metrohm

Dibutyl phthalate

Procedure

12.5 g of silica are introduced into the kneader of the Brabenderplastograph. With continued mixing (kneader paddle speed 125 rpm),dibutyl phthalate runs into the mixture at a rate of 4 ml/minute. Theforce required for incorporation is low. Toward the end of thedetermination, the mixture becomes poorly free-flowing. This fact isdocumented in an increase in the required force, which is indicated onthe scale. When the scale has moved by 300, DBP metering isautomatically shut off.

Evaluation

The density of DBP is 1.047 g/ml. The DBP absorption is based on theanhydrous, dried substance. When using precipitated silicas ofrelatively high moisture content, the value must be corrected using thefollowing table if these silicas are not dried prior to thedetermination of the DBP number.

Correction table for dibutyl phthalate absorption - anhydrous - % water% water % water .0 .2 .4 .6 .8 0 0 2 4 5 7 1 9 10 12 13 15 2 16 18 19 2022 3 23 24 26 27 28 4 28 29 29 30 31 5 31 32 32 33 33 6 34 34 35 35 36 736 37 38 38 39 8 39 40 40 41 41 9 42 43 43 44 44 10 45 45 46 46 47

The correction figure corresponding to the water content is added to theexperimentally determined DBP value; for example, a water content of5.8% would mean an add-on of 33 g/100 g for the DBP absorption.

1.3 Particle Size

The particle size is determined using a Malvern Mastersizer in ethanolfollowing ultrasound treatment for 5 minutes. The measurement is madeautomatically and provides the average particle size d_(4.3) from avolume distribution.

1.4 Determination of the Tristimulus Value R_(y) in Accordance with DIN5033

Application

Using the Datacolor 3890 spectrophotometer, the tristimulus value R_(y)is determined for silicas, silicates, and zeolites (powder suspensions).

Analytical Procedure:

The silica to be analyzed is first ground to an average particlediameter of about 8 to 15 μm and then pressed to a tablet using a powderpress. The amount required depends on the fineness of the powder. Theamount of powder introduced is such that the thread of the press closurereaches its last turn.

The samples are placed under the meter, and whiteness measurement R_(y)and R₄₆₀ are selected from the menu of the control computer. After thesample designation has been entered, the space key is operated in orderto start the measurement.

Following entry of the memory code, the measurements are printed out.

The values are calculated automatically in accordance with the followingformula:

$y = {\sum\limits_{400}^{700}\;{{S(\lambda)}*{Y(\lambda)}*{R(\lambda)}}}$where

-   Y(λ) is the standard distribution coefficient,-   S(λ) is the relative spectral radiation distribution of the    illumination source, and-   R(λ) is the spectral reflectance of the sample.    1.5 Determination of the Sears Number of Silicas, Silicates and    Hydrophobic Silicas    1. Application:

Free OH groups are detectable by titration with 0.1 N KOH in the rangefrom pH 6 to pH9.

2. Apparatus

-   -   2.1 Precision balance accurate to 0.01 g    -   2.2 Memotitrator DL 70, Mettler, equipped with 10 ml and 20 ml        Burette, 1 pH electrode and 1 pump (e.g., NOUVAG pump, type SP        40/6)    -   2.3 Printer    -   2.4 Titration vessel 250 ml, Mettler    -   2.5 Ultra-Turrax 8000–24000 rpm    -   2.6 Thermostated waterbath    -   2.7 2 dispensers 10–100 ml for metering methanol and deionized        water    -   2.8 1 dispenser 10–50 ml for metering deionized water    -   2.9 1 measuring cylinder 100 ml    -   2.1 OIKA universal mill M 20        3. Reagents    -   3.1 Methanol p.A.    -   3.2 Sodium chloride solution (250 g NaCl p.A. in 1000 ml        deionized water)    -   3.3 0.1 N hydrochloric acid    -   3.4 0.1 N potassium hydroxide solution    -   3.5 Deionized water    -   3.6 Buffer solutions pH 7 and pH 9        4. Procedure    -   4.1 Sample preparation        -   Grind about 10 g of sample for 60 seconds in the IKA            universal mill M 20.        -   Important: Since only very finely ground samples give            reproducible results, these conditions must be strictly            observed.    -   4.2 Analytical procedure        -   4.2.1 Weigh out 2.50 g of the sample prepared in accordance            with section 4.1 into a 250 ml titration vessel.        -   4.2.2 Add 60 ml of methanol p.A.        -   4.2.3 After complete wetting of the sample, add 40 ml of            deionized water        -   4.2.4 Disperse for 30 seconds using the Ultra-Turrax at a            speed of about 18000 rpm        -   4.2.5 Rinse particles of sample adhering to the vessel edge            and stirrer into the suspension using 100 ml of deionized            water        -   4.2.6 Condition sample to 25° C. in a thermostated waterbath            (for at least 20 minutes)        -   4.2.7 Calibrate pH electrode with the buffer solutions pH 7            and pH 9        -   4.2.8 The sample is titrated in the Memotitrator DL 70 in            accordance with method S 911. If the course of titration is            indistinct, a duplicate determination is carried out            subsequently.            The Results Printed out are as Follows:

pH V₁ in ml/5 g V₂ in ml/5 g5. Calculation:

$V_{1} = \frac{V*5}{E}$ $V_{2} = \frac{V*5}{E}$

-   V₁=ml KOH or ml HCl to pH6/5 g of substance-   V₂=ml KOH consumed to pH9/5 g of substance-   E=initial mass    Principle:

First of all the initial pH of the suspension is measured, thenaccording to the result the pH is adjusted to 6 using KOH or HCl. Then20 ml of NaCl solution are metered in. The titration is then continuedto a pH of 9 using 0.1 N KOH.

Sears numbersSi—OH+NaCl→Si—ONa+HClHCl+KOH→KCl+H₂O1.6 Determination of the Tamped Density in Accordance with DIN/ISO787/11Procedure:

10 g of the sample under analysis are weighed accurately to 0.01 g onthe precision balance, and are introduced into the graduated 250 mlglass cylinder of the jolting volumeter. After 1250 jolts, the volume ofthe tapped material is read off.

Calculation:

${{Tapped}\mspace{14mu}{{density}:{g\text{/}1}}} = \frac{E \cdot 1000}{I}$

-   E=initial mass in g-   I=volume in ml    Apparatus:

Precision balance Engelsmann, Ludwigshafen Jolting volumeter 250 mlglass cylinder, Engelsmann, Ludwigshafen graduatedRemarks

In special cases, the material may be passed through a 500 μm sievebefore weighing, or the initial mass may be increased. This must bespecified in a test report.

1.7 Determination of CTAB Surface Area

1. Application

The method is based on the adsorption of CTAB(N-cetyl-N,N,N-trimethylammonium bromide) on the “external” surface,which is also referred to as the “rubber-active surface”.

The adsorption of CTAB takes place in aqueous solution at pH=9 withstirring and ultrasound treatment. Excess, unadsorbed CTAB is determinedby back-titration with SDSS (dioctylsodium sulfosuccinate solution)using a titroprocessor, the endpoint being given by the maximum cloudingof the solution and determined using a phototrode.

For the calculation, an occupancy of 0.35 nm² per CTAB molecule isassumed.

The determination is made in accordance with ASTM 3765.

With each measurement series, a standard sample of type VN 3 silicashould be tested as well.

2. Reaction Equation: (Back-Titration)

$\underset{NDSS}{R_{1} - {SO}_{3}^{-}} + {\left. \underset{CTAB}{{{\,^{+}N}\left( {CH}_{3} \right)}_{3}R_{2}}\longrightarrow R_{1} \right.{SO}_{3}{N\left( {CH}_{3} \right)}_{3}R_{2}}$3. Apparatus:

-   -   3.1 Mill, e.g. IKA, type: M 20    -   3.2 Analytical balance    -   3.3 Magnetic stirrer    -   3.4 Magnetic stirrer rod    -   3.5 Titroprocessor, e.g., METTLER, type DL 55 or DL 70, equipped        with:        -   pH electrode, e.g., Mettler, type DG 111        -   phototrode, e.g. Mettler, type DP 550, and        -   burette, 20 ml volume, for SDSS solution,        -   burette, 10 ml volume, for 0.1 N KOH    -   3.6 titration beakers, 100 ml, made of polypropylene    -   3.7 glass titration vessel, 150 ml volume, closable with snap-on        lid    -   3.8 conical flasks, 100 ml volume, closable with screw lid or NS        stopper    -   3.9 ultrasound bath    -   3.10 pressure filtration device    -   3.11 membrane filter of cellulose nitrate, pore sizes of 0.1 μm,        47 mm Ø, e.g., Sartorius type 113 58    -   3.12 pipettes, 5 ml, 100 ml        4. Reagents:    -   4.1 Potassium hydroxide solution, 0.1 N    -   4.2 CTAB solution, 0.0151 mol/l        -   5.50 g of CTAB are dissolved with stirring (magnetic            stirrer) in about 800 ml of warm (about 30–40° C.)            demineralized water in a glass beaker, transferred to a 11            graduated flask, made up to the mark with demineralized            water after cooling to 23–25° C., and transferred to a stock            bottle.        -   Note:        -   The solution must be stored and the measurement conducted at            ≧23° C., since CTAB crystallizes out below this temperature.            The solution should be prepared 10–14 days prior to use.    -   4.3 SDSS solution 0.00426 mol/l        -   1.895 g of SDSS (dioctylsodium sulfosuccinate) in a glass            beaker are admixed with about 800 ml of demineralized water            and the mixture is stirred with a magnetic stirrer until all            of the material has dissolved. The solution is then            transferred to a 11 graduated flask, made up to the mark            with demineralized water, and transferred to a stock bottle.        -   SDSS solution readily undergoes biodegradation. The solution            prepared should therefore be sealed well and should not be            stored for more than 3 months.        -   The concentration of the CTAB solution is assumed to be            exact: 0.0151 mol/l.        -   The concentration of the SDSS solution should be determined            daily by means of a “blank” titration.            5. Procedure:    -   5.1 Blank titration (to determine the concentration of the SDSS        solution)    -   5.2 The consumption of SDSS solution for 5 ml of CTAB solution        should be checked (blank value) 1× per day before each series of        measurements    -   5.1.2 Pipette precisely 5 ml of CTAB solution into titration        beakers    -   5.1.3 Add about 50 ml of demineralized water    -   5.1.4 Titrate with the titroprocessor until the end of titration        -   Each blank titration should be performed as a duplicate            determination; in the case where values do not agree,            further titration should be carried out until the results            are reproducible.    -   5.2 Adsorption    -   5.2.1 The granulated and coarse samples are ground in a mill        (the beater blade of the mill must be covered)    -   5.2.2 Weight out exactly 500 mg of the ground sample on the        analytical balance to a precision of 0.1 mg    -   5.2.3 Transfer the sample amount weighed out quantitatively to a        150 ml titration vessel with magnetic stirrer rod    -   5.2.4 Add exactly 100 ml of CTAB solution, seal titration vessel        with lid, and stir on a magnetic stirrer for 15 minutes    -   5.2.5 Screw the titration vessel onto the titroprocessor and        adjust the pH of the suspension to 9.0±0.05 using KOH, 0.1 mol/l    -   5.2.6 4-minute treatment of the suspension in the ultrasound        bath    -   5.2.7 Filtration through a pressure filter fitted with a        membrane filter.        -   During adsorption, it must be ensured that the temperature            is held within the range from 23° C. to 25° C.    -   5.3 Titration    -   5.3.1 Pipette 5 ml of filtrate (see section 5.2.7) into 100 ml        titration beakers and make up to about 50 ml with demineralized        water    -   5.3.2 Screw titration beakers onto the titrator    -   5.3.3 Carry out titration with SDSS solution in accordance with        the defined measurement method, until clouding reaches a        maximum.        -   Each titration should be performed as a duplicate            determination; in the case where values do not agree,            further titration should be carried out until the results            are reproducible.            6. Calculation

${m^{2}\text{/}g} = {\left( {V_{1} - V_{2}} \right)*\frac{100*E*2*578.435}{V_{1}*1000}}$${m^{2}\text{/}g} = {\left( {V_{1} + V_{2}} \right)*\frac{115.687*E}{V_{1}}}$${m^{2}\text{/}g} = {\left( {V_{1} + V_{2}} \right)*\frac{115.687}{V^{1}}*5.5}$

-   V₁=blank sample (ml of SDSS when using 5 ml of CTAB)-   V₂=consumption (ml of SDSS when using 5 ml of filtrate)-   E=initial mass g CTAB/1 (5.5 g)-   578.435=occupancy of 1 g of CTAB in m².    The measurement is normally to be given corrected to the anhydrous    substance:

${m^{2}\text{/}g} = \frac{{CTAB}\mspace{11mu} m^{2}\text{/}g*100}{100 - {\%\mspace{20mu} H_{2}O}}$

Where the measured value for the standard sample differs by more than±3²/g from the theoretical value, the entire measurement series must berepeated.

7. Notes

-   re 1. In the literature, SDSS (dioctylsodium sulfosuccinate) is also    called Aerosol OT.    -   On samples with a pH >9, such as Extrusil, the pH is measured        but not corrected, since the acid may alter the surface.    -   Prior to beginning the titration, the phototrode is set to 1000        V, corresponding to a transparency of 100%.-   re 3. For measuring the different prescribed volumes of the CTAB    solution, it is also possible to use dispensers or piston-stroke    pipettes, provided they are regularly calibrated.-   re 4. The solutions indicated in sections 4.1 and 4.3 can also be    purchased as ready-to-use solutions. The present supplier is Kraft,    Duisburg.    -   Telephone: 0203–58-3025.        -   Order No. 6056.4 CTAb solution 0.0151 ml/1        -   Order No. 6057.4 SDSS solution 0.00423 mol/l (in 2.5-liter            glass bottles)-   re 5.2.4 Hydrophobic samples which are not wetted after stirring are    to be dispersed carefully using an ULTRA-TURRAX before the pH is    adjusted, in order to wet them.-   re 5.2.5 For adjusting the pH it is advisable to use a titrator. The    titration is carried out in accordance with the endpoint method.-   re 5.2.7 For filtration, nitrogen from a gas bottle is to be used;    an admission pressure of 4–6 bar is to be set.-   re 6. Should it be necessary to repeat a measurement series, it    should be noted in particular that the pH meter used to set the pH    must also be recalibrated.    1.8 Determination of Water Vapor Absorption (Water Vapor Isotherms)

To determine the water vapor absorption, the sample is exposed todifferent relative humidities at constant temperature (30° C.). Theestablishment of a constant weight is awaited.

To start with, completely dry air (i.e., air humidity approximatelyzero) is used. After the equilibrium weight has been reached, thisweight is chosen as the reference point; in other words, the water vaporabsorption at a higher air humidity is expressed as the differencebetween the sample weight in completely dry air (following establishmentof equilibrium) and the sample weight in humid air (followingestablishment of equilibrium). The air humidity is varied in steps of10%.

In order to rule out hysteresis effects, both the water adsorption andthe water vapor desorption are measured.

Example 1

50.0 m³ of water are charged to a reaction vessel. Slowly, 9.2 m³ ofwaterglass solution and 0.9 m³ of H₂SO₄ are added with stirring to theinitial charge, an alkaline pH being maintained in the mixture duringthe addition. After the end of the addition of waterglass and H₂SO₄, thepH of the resulting suspension is within the alkaline range. Thesuspension is acidified and filtered, and the solid product is washedwith deionized water. The hydrophilic base silica can be dried,preferably by an accelerated drying method. The following data relate tothe dried precipitated silica thus obtained.

BET surface area (m²/g) 150–170 CTAB surface area (m²/g) 150–170 Loss onignition based on the substance dried 3 ± 0.5 2 h/105° C. (DIN 55921)(%) pH 5% (methanol/aqueous solution) (DIN 53200)  6–7 Conductivity (in5% aqueous dispersion) (μS) <150 Tapped density (g/l) >250 Sears number <13

Example 2

The hydrophobic base silica was coated dry with silicone oil(methyl-terminated polydimethylsiloxane, viscosity 50 Pas, e.g., DOWCORNING (R) 200 FLUID 50 CS, carbon content about 33%) and conditionedby aging at room temperature for more than 48 hours (3 days) until ithad obtained a methanol wettability of at least 20%. The material isheat-treated under oxidizing conditions in a muffle furnace with a bedheight of not more than 2 m at a temperature above 330–360° C. for 1 h.The analytical data of the resulting material are given in Table 1.1.

TABLE 1.1 Product data, oxidatively heat-treated Water % 1.1 Loss onignition 4.3 N₂ surface area m²/g 89 pH 7.9 Conductivity μS 40 DBP % 189C content % 3.9 Reflectance % 95 Methanol wettability % 67

Example 3

The base silica is coated with silicone oil (methyl-terminateddimethylpolysiloxane, 50 Pas, e.g. DOW CORNING (R) 200 FLUID 50 CS,carbon content approximately 33%) in a Gericke mixer in a mass ratio of1:1. The resulting powder is conditioned for an hour at a temperature of105° C. This produces a material in which, although it is wettable bywater, the silica and silicone oil can no longer be separated from oneanother in water. Mixing of this masterbatch with filtercakes of thebase silica produces stable formulations in which the silicone oil nolonger separates from the hydrophilic filtercake. A hydrophobicmasterbatch and a hydrophilic filtercake thus prepared (solids contentabout 20%) are conveyed together into the spin-flash drier, in whichthey are mixed (in a mass ratio of about 1:12) and dried. Thedimethylsiloxane-treated silica is conditioned by aging at roomtemperature for at least 48 hours (about 3 days) until it has attained amethanol wettability of at least 20%. The analytical data of theconditioned silica are given in Table 2.1.

TABLE 2.1 Analytical data of the conditioned silica Water % 3.4 pH 6.3Conductivity μS 100 N₂ surface area m²/g 74 CTAB surface area m²/g 119DBP absorption g/100 g 198 Tapped density g/L 323 Reflectance % 95.9 Ccontent % 5.03 Methanol wettability % >20

The conditioned precipitated silica is heat-treated under oxidizingconditions in a muffle furnace with a bed height of not more than 2 cmat a temperature above 330–360° C. for 1 h. The analytical data of theoxidatively heat-treated precipitated silica are given in Table 2.3.

TABLE 2.3 Product data, oxidatively heat-treated material N₂ surfacearea m²/g 96 CTAB surface area m²/g 41 Reflectance % 94.5 C content %3.93 Methanol wettability approx. % 67

Example 4

The initial distribution of silicone oil (viscosity of 50 Pas,methyl-terminated, e.g., DOW CORNING (R) 200 FLUID 50 CS, carbon contentapproximately 33%) on the silica (in the form of filtercakes) takesplace in a spin-flash drier, with simultaneous drying. Thedimethylsiloxane-treated silica is conditioned by aging at roomtemperature for at least 48 hours (about 3 days) until it has attained amethanol wettability of at least 20%. The analytical data of theconditioned silica are given in Table 3.1.

TABLE 3.1 Analytical data of the conditioned silica Water % 5.2 pH 6.1Conductivity μS 41 N₂ surface area m²/g 84 CTAB surface area m²/g 132Tapped density g/L 317 Reflectance % 95.9 C content % 4.12 Methanolwettability % >20

The material is heat-treated under oxidizing conditions in a mufflefurnace with a bed height of not more than 2 cm at a temperature above330–360° C. for 1 h. The analytical data of the oxidatively heat-treatedprecipitated silica are given in table 3.2.

TABLE 3.2 Product data, oxidatively heat-treated material N₂ surfacearea m²/g 102 Reflectance % 94.7 C content % 3.59 Methanol wettabilityapprox. % 67

Example 5

10 The base silica is coated in a mass ratio of 1:1 with silicone oilDOW CORNING (R) 200 FLUID 50 CS (dimethylpolysiloxane, 50 Pas,terminated with trimethylsilyl groups, carbon content approximately33%). The resulting powder is conditioned for an hour at a temperatureof more than 100° C. This produces a material in which, although it iswettable by water, the silica and silicone oil can no longer beseparated from one another with water. Mixing of this masterbatch (i.e.,50% silica and 50% silicone oil) with aqueous silica dispersionsproduces stable suspensions in which the silicone oil can no longer beseparated from the silica. The overall mixture typically contains 1 partby weight of silicone oil, about 4–8 parts by weight of silica, and20–60 parts by weight of water. To prepare such a suspension, themasterbatch (i.e., 50% silica and 50% silicone oil) is mixed thoroughlywith about 14–16 times the amount of filtercake (solids contentapproximately 20%) and about 11–13 times the amount of additional water.The analytical data of the dispersion are given in Table 4.1, those ofthe correspondingly conditioned silica in Table 4.2.

TABLE 4.1 Suspension data Solids content % 12.8 PH 5.2 Conductivity μS382 Viscosity mPas 183

The suspension is dried using a spray drier. Thedimethylsiloxane-treated silica is conditioned by aging at roomtemperature for at least 48 hours (about 3 days) until it has attained amethanol wettability of at least 20%. The analytical data of theconditioned silica are given in Table 4.2.

TABLE 4.2 Analytical data of the conditioned silica Loss on ignition %12.4 Water % 2.2 pH 6.4 Conductivity μS 135 N₂ surface area m²/g 80 CTABsurface area m²/g 131 DBP absorption g/100 g 236 Tapped density g/L 256Reflectance % 94.5 C content % 4.21 Methanol wettability % >20%

The conditioned precipitated silica is heat-treated under oxidizingconditions in a muffle furnace with a bed height of not more than 2 cmat a temperature above 330–360° C. for 1 h. The analytical data of theoxidatively heat-treated precipitated silica are given in Table 4.3.

TABLE 4.3 Product data, oxidatively heat-treated Water % 0.6 N₂ surfacearea m²/g 91 CTAB surface area m²/g 52 Reflectance % 94.3 C content %4.01 Methanol wettability approx. % 66

Example 6

Silicone oil, (polydimethylsiloxane, 50 Pas, e.g., DOW CORNING (R) 200FLUID 50 CS, carbon content approximately 33%) is suspended in asuspension of the base silica (solids content 12.8%) using a high-shearmixer. The distribution of the silicone oil in the silica suspension isconserved by immediate spray drying. The dimethylsiloxane-treated silicais conditioned by aging at room temperature for at least 48 hours (about3 days) until it has attained a methanol wettability of at least 20%.The analytical data of the conditioned silica are given in Table 5.1.

TABLE 5.1 Analytical data of the conditioned silica Loss on ignition %13.0 Water % 2.2 pH 5.5 Conductivity μS 100 N₂ surface area m²/g 85 CTABsurface area m²/g 137 DBP absorption g/100 g 253 Tapped density g/L 270Reflectance % 94.2 C content % 4.78 Methanol wettability % >20%

The material is heat-treated under oxidizing conditions in a mufflefurnace with a bed height of not more than 2 cm at a temperature above330–360° C. for 1 h. The analytical data of the oxidatively heat-treatedprecipitated silica are given in Table 5.2.

TABLE 5.2 Product data, oxidatively heat-treated Water % 1.6 N₂ surfacearea m²/g 102 CTAB surface area m²/g 43 Reflectance % 94.2 C content %3.44 Methanol wettability approx. % 65

The disclosure of the priority document, German Patent Application No.101 38 490.4, filed Aug. 4, 2001, is incorporated by reference herein inits entirety.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

1. A process for preparing hydrophobic precipitated silica, the processcomprising: a) preparing a mixture of an organopolysiloxane derivativeand a precipitated silica; b) conditioning the mixture at from 100 to150° C. for a period of from 0.5 to 2 hours; and c) conducting oxidativeheat treatment at more than 300° C. with an oxidizing gas.
 2. Theprocess as claimed in claim 1, wherein in step a) the preparingcomprises adding the organopolysiloxane derivative to the precipitatedsilica; and in step a) the precipitated silica has a water content offrom 1.0 to 80%.
 3. The process as claimed in claim 1, wherein in stepa) the preparing comprises adding the organopolysiloxane derivative tothe precipitated silica; in step a) the precipitated silica has a watercontent of from 70 to 99%; and step a) further comprises isolatingsolids from water.
 4. The process as claimed in claim 1, wherein theoxidizing gas comprises at least one selected from the group consistingof Cl_(2,) N₂O, NO, NO_(2,) N₂O₅, O₃, O_(02,) Br₂ and F₂.
 5. The processas claimed in claim 1, wherein the oxidizing gas further comprises aninert gas.
 6. The process as claimed in claim 1, wherein the oxidizinggas is air or a mixture of an inert gas with air.
 7. The process asclaimed in claim 1, wherein the oxidizing gas comprises 99% or less byvolume of at least one inert gas.
 8. The process as claimed in claim 1,wherein the oxidizing gas comprises up to 80% by volume of water.
 9. Theprocess as claimed in claim 1, wherein one of steps a), b), and c) iscarried out a number of times in succession.
 10. The process as claimedin claim 1, wherein steps a) and b) are carried out a number of times insuccession.